The present invention relates to rotating machinery and, more particularly, to bearings for high-speed rotors in rotating machinery.
High-speed rotors are used in, for example, steam turbines, gas turbines and compressors where they are subjected to forces which may produce vibrations capable of leading to failure of the machine. One such force excitation source is rotor imbalance. Rotor imbalance produces synchronous vibrations at the rotational frequency of the rotor. These vibrations are amplified when the rotor speed passes through specific narrow ranges of speed called critical speeds or system resonances. The rotor vibrational amplitudes which occur during transition through these critical speed zones may be limited by improved rotor balance, additional system damping and by reducing the time that the rotor spends in the critical zones as it accelerates or decelerates therethrough. A well-balanced rotor which does not have a critical speed corresponding to its operating speed may therefore reach and operate at speeds substantially higher than the critical speeds without exhibiting excessive synchronous vibrations.
In contrast to synchronous vibrations, sub-synchronous vibrations are not produced by rotor imbalance and their occurrence is not limited to narrow speed zones. Thus, sub-synchronous vibrations are not generally curable by improved rotor balance nor by rapid acceleration or deceleration through specific speeds. A machine subject to sub-synchronous vibrations typically exhibits a pattern in which the vibration develops suddenly when the rotor speed exceeds a threshold value. The frequency of the vibration is usually a fraction of the rotor speed and often coincides with a system resonant frequency which has been traversed as the rotor speed is increased to the threshold value. The amplitude of the sub-synchronous vibration component of total vibration often exceeds the synchronous component of vibration due to rotor imbalance. Further increases in rotor speed above the threshold value generally do not change the sub-synchronous vibration frequency but such increases do increase the amplitude thereof. Thus, the rotor threshold speed at which sub-synchronous vibrations begin may be a limiting speed for the rotor and may force operation at lower than optimum design speeds.
Sub-synchronous rotor vibrations are produced by phenomena of a different nature than rotor imbalance. The phenomena share a common characteristic: displacement of the rotor in a given radial direction produces two distinct forces; one of these is a restoring force acting in the direction of rotor displacement and the other is normal to it. The latter force is known as a cross-coupling force.
Practical experience and theoretical considerations have identified the major phenomena producing such cross-coupling forces which excite sub-synchronous vibrations. These phenomena include sliding friction in rotor shrink fits and couplings, stress-strain hysteresis in rotor materials, hydrodynamic bearing behavior and aerodynamic phenomena produced by labyrinth seals and non-uniform blade tip clearances in turbines and compressors. Several such cross-coupling forces may exist on a given rotor.
While the sources of cross-coupling forces are known to experienced rotor designers, it may be impossible to avoid all of them by practicable design modifications or to estimate their magnitude at the design stage. Some steps which have successfully been employed to reduce cross-coupling forces include eliminating shrink fits wherever possible, reducing the opportunity for shrink fits, which cannot be eliminated, to open and permit frictional sliding, minimizing circumferential variations in blade tip clearances, and using fluid-film bearing designs which minimize cross-coupling forces from this source. Tilting-pad journal bearings are widely used as an alternative to cylindrical bore bearings because of their ability to minimize bearing-induced cross-coupling forces.
Despite employment of these well-known design steps, destructive sub-synchronous rotor vibrations continue to exist as a serious problem on many high-speed rotating machines. Further design approaches to counteract the destabilizing effects of cross-coupling forces are clearly desirable. Often, the problem of sub-synchronous vibrational instability on a given new machine is not suspected until the first tests reveal it. The appearance of an unsuspected sub-synchronous vibration condition makes it particularly desirable to have a simple solution which can be applied without extensive rotor or stator modifications to counteract the destabilizing forces.
In 1963, a paper: W. Kellenberger, 50 The Brown Boveri Rev. No. 11/12, Nov/Dec 1963, pp. 756-766; suggested that the effects of cross-coupling forces may be minimized by supporting a rotor in a manner which employs different stiffnesses in the horizontal and vertical directions.
Other writers have suggested the possibility of using anisotropy in a support bearing, but none have advanced any particular structure which accomplishes this end, e.g. J. C. Nicholas, et al, ASME Spec. Publ. Topics in Fluid-Film Bearing System Design and Optimization, 1978, pp. 55-78, "The Influence of Tilting Pad Bearing Characteristics on the Stability of High Speed Rotor-Bearing Systems".
Although the advantages of anisotropic bearing support appears to be established by the referenced papers, the literature is innocent of any suggestion of an embodiment of a bearing system for using anisotropic bearing support in high-speed machinery.